Hitherto, oil impregnated sintered bearings have been used in motors for automotive use (electrical components), home appliances (air conditioners, refrigerators, etc.), audio equipments (CD players, MD players, etc.) and, in recent years, due to the rapid spread of computers (motors for memory devices) and mobile telephones (vibration motors), there has been an increasing demand for oil impregnated sintered bearings. The commercialization of fluid dynamic bearings is also being considered to reduce vibration over a wide rotational range. Furthermore, the increasingly large bearing loads resulting from the recent trend toward smaller and thinner equipments and higher motor speeds, have led to demands for improved bearing materials and bearing lubricating oils with higher performance.
It has been disclosed that lubricating oils containing a synthetic hydrocarbon oil such as a poly-α-olefin or an ester oil such as a dibasic acid diester or a neopentyl polyol ester exhibit excellent performance as lubricating oils for oil impregnated sintered bearings or fluid dynamic bearings (Japanese Unexamined Patent Publications Nos. 1995-53984, 1997-125086, and 1999-172267). However, these lubricating oils cannot satisfactorily withstand severe operating conditions, and therefore lubricating oils that exhibit further improved characteristics are demanded.
Specifically, characteristics required for such a bearing lubricating oil include excellent heat resistance (anti-oxidation stability, evaporation resistance, small change in viscosity), applicability over a wide temperature range, excellent lubricating ability, absence of influence on the bearing materials themselves, etc. Among these, considerable importance is being placed on heat resistance due to large temperature elevation caused by increased loads on bearings.
Furthermore, reduced electrical power consumption is also demanded in order to reduce emission of carbon dioxide which causes global warming. Therefore, to reduce energy losses caused by viscous friction, lubricating oils for bearings are required to have a low viscosity over a wide temperature range and low friction. Generally, however, with a decrease in the viscosity of lubricating oils, heat resistance, and in particular evaporation resistance, tends to become poor, and therefore no lubricating oils with sufficient energy-saving capabilities for oil impregnated sintered bearings or fluid dynamic bearings have been realized.